The alien epoxy comes in tubes and, when used, restores the condition of the player's currently-equipped weapon. The weapon must be equipped in order for the epoxy to repair it, therefore alien epoxy cannot repair broken weapons. While the effect of one tube may not completely repair a weapon, applying multiple tubes one after the other (regardless of Repair skill) can increase a weapon's condition to 100%.
Since normal "Earth-made" weapons are rare aboard the ship, alien epoxy is an excellent alternative to using salvaged weapons for repair. It also has no weight allowing them to be carried en masse.